1. Field of the Invention
The present invention relates in general to a method of converting an analog signal into a digital signal, and more particularly to a novel conversion method of obtaining a digital angle output using a negative feedback control system having a resolver within a closed loop to enhance conversion performance (such as resolution, accuracy, high speed follow-up property and noise resistance) to thereby make it possible to provide an inexpensive and high-performance resolver/digital (R/D) conversion unit integrated resolver used in fields of automobiles, robots, factory automation (FA) and household electric appliances).
2. Description of the Related Art
As for a method of converting into a digital signal a resolver output (analog signal) from a resolver of this sort which has been conventionally used, for example, it is possible to give a tracking method which is disclosed in JP 3,442,316 B and U.S. Pat. No. 6,278,388 and which is established as the related art.
Since the conventional method of converting an analog signal into a digital signal is constituted as described above, the following problems are encountered.
In the tracking method (see JP 3,442,316 B and U.S. Pat. No. 6,278,388) as the conventional conversion method, a closed loop is configured with first and second amplitude modulation signals outputted from a rotation detector (resolver) as a starting point. Hence, the rotation detector (resolver) itself is independent of a control system, and thus it is difficult to enhance the stability and the safety for the conversion operation, and the property of suppression of disturbances such as noises.
In addition, means for writing desired nonlinear data in a read only nonvolatile memory (ROM) in advance, and so forth is used in order to obtain the nonlinear characteristics of a SIN signal and a COS signal, and hence a chip area larger than that of other circuits is occupied also depending on a ROM capacity. Thus, a chip area cannot be made small when circuits are integrated with one another in the form of an IC, which leads to an increase in cost.
In addition, in the above-mentioned tracking method, it is difficult to maintain the high speed follow-up property unless the resolution is reduced.
Also, in addition to the above-mentioned tracking method, a method including extracting a digital angle signal Φ from phase information of an exciting component is conventionally known as a phase method. In this method, however, it is difficult to obtain a continuous output in real time with respect to a rotational angle input.